Voltage controlled oscillator (VCO) converts an external voltage into a desired oscillation frequency. The VCO has a great influence on the efficiency and stability in modulation/demodulation of RF signals.
Generally, the VCO uses a dielectric resonator, which has a good stability, is less influenced by temperature change, and has a high quality factor. In recent years, as RF components are required to be small-sized, lightweight and economic, a planar resonator suitable for Monolithic Microwave Integrated Circuit (MMIC) such as Hair-Pin resonator is widely used.
FIG. 1 is a circuit diagram of a conventional VCO. In the VCO of FIG. 1, a varactor diode is added to a single oscillator in parallel.
Referring to FIG. 1, the conventional VCO includes a resonator 110 having upper and lower portions connected to transmission lines, a varactor diode 120 for converting a frequency in the resonator 110, an active element 130, a band pass filter 140 for DC shielding, and an open stub 150 for generating a negative resistance.
The resonator 110 has a band pass filter characteristic, and coupling gaps 160 and 170 exist in the upper and lower portions of the resonator 110. The conventional VCO of FIG. 1 changes the oscillation frequency by adjusting the length between the resonator 110 and the active element 130 and the length of the open stub 150.
In the VCO widely used in RF/microwave band, the open stub 150 is connected to a source terminal of the active element 130 in order to generate the negative resistance. To control the negative resistance, a pad is added around the open stub 150, or the length of the open stub 150 is adjusted little by little.
The conventional VCO has a poor quality factor characteristic and generate great error in adjusting the negative resistance. Further, the MMIC resonator cannot adjust the negative resistance after its fabrication is completed.